ORIGIN AND AGE OF FLUIDS AT THE CERRO PABELLÓN GEOTHERMAL SYSTEM, NORTHERN CHILE

Abstract

The Cerro Pabellón geothermal system in the Central Andean Volcanic Zone hosts the first geothermal power plant in South America (48 MW), and is located in northern Chile at 4500 m a.s.l. The geothermal area is hosted within a NW-SE graben structure, and is defined as a blind high-enthalpy geothermal system. Here we present the results of a geochemical and isotopic study that aims to understand the origin and age of the geothermal fluids at Cerro Pabellón. Brine and condensate samples obtained from production wells were analyzed for ?D, ?18O and ?13C. In addition, apparent ages of the geothermal fluid samples were constrained using 3H and 14C methods. ?D-?18O values indicate a meteoric origin of fluids, which probably underwent fluid-rock interaction at temperatures of about 250°C. An andesitic water component can be inferred considering a lighter meteoric endmember. ?13C values are also consistent with a magmatic input on fluids for the gaseous components. Tritium (3H) and radiocarbon (14C) analyses were carried out to assess the age and residence time of geothermal fluids. Taking into account the inherent limitations of these methods, a probable age of at least several thousands of years is estimated. These results should be expanded and complemented with further data to better understand the recharge dynamics of the Cerro Pabellón system and evaluate the long-term sustainability of the power plant.